Amorphous alloy thin films of rare-earth transition metals have been used in erasable photomagnetic memory media. Especially, an amorphous alloy of terbium, iron, and cobalt (TbFeCo), when it is magnetized, exhibits a reasonable photomagnetic effect and large vertical magnetic anisotropy aligned perpendicularly to the film surface. Thus the amorphous alloy of TbFeCo has been thought of as an advantageous photomagnetic memory material.
Techniques of forming such thin films includes vacuum deposition and sputtering. Particularly, the sputtering is widely used because it produces good adhesion between a substrate and the thin film formed thereon.
Sputtering is a method in which sputtered atoms extracted from a solid into space due to exchange of momentum build up on a substrate when the accelerated particles impinge the surface of the solid and thus form a solid.
The TbFeCo amorphous alloy thin film obtained by, for example, a magnetron sputtering apparatus as shown in FIG. 13 is formed of an initial layer and a columnar crystal net, and fine particles (FIG. 7). This magnetron sputtering apparatus is a diode sputtering apparatus whose sputtering source is modified. As shown in FIG. 13. a magnet 8 is disposed under a target 5 to form a tunnel-shaped magnetic field of 0.02 to 0.05 T (200 to 500 gauss). Electrons e are trapped within the magnetic field and then turn in a spiral fashion above the target 5 along the magnetic field to produce an Ar gas plasma. The particles of the Ar plasma collide with the target to produce the sputtered atoms. Formation of a columnar crystal structure of TbFeCo film by sputtering has not yet been fully explained. It is generally thought that this film is of the Volmer-Weber type. The columnar crystal growth is considered to occur around nuclei produced at an adsorption site having a large adsorption energy of atoms due to motion of the sputtering atoms on the surface of the substrate. The number of sites is much less than the number of the surface atoms. Thus, the structure of the thin film is island-like or patch-like at an early stage of deposition of thin film (FIG. 12(a)). If the substrate is amorphous, then the surface having the highest density of crystal is parallel to the surface of the substrate in many cases. If every grain boundary extends from the substrate to the top surface of the film, then a columnar crystal structure is produced. When the width of the island grows to contact adjacent islands, lateral growth of the island stops (FIG. 12(b)). Although side surfaces of the islands are usually different from each other, it is difficult for realignment to occur if the mobility of the condensed sputtered atoms is small Thus the grain boundaries are formed as they are. The sputtered atoms are considered to arrive from the space above the islands in view of the fact that there are gaps between the islands, but the Ar discharge gas atoms are actually scattered in random directions. Therefore, the number of sputtered atoms that are incident in directions oblique to the normal direction to the substrate will increase. These islands disturb the atoms which would otherwise go into shadowed portions of the islands or gaps between the islands (shadowing effect). In view of this effect, taller portions of the islands grow disproportionately to thereby form columnar crystal (FIG. 12(c)). Consequently, the ratio of the number of atoms of discharge Ar gas to the number of the sputtered atoms increases to eventually increase the number of atoms of the discharge Ar gas which are captured by the grain boundary, thereby producing voids in addition to the ordinary grain boundaries. Reference is made to "Sputtering Phenomena" by Akira Kanahara, Tokyo University Publishing Committees.
The following factors are considered to contribute to formation of the columnar crystal structure.
1) Thin film growth is of the Volmer-Weber type.
2) Fibrous structure or epitaxial orientation growth occurs easily and grows rapidly in a particular direction.
3) Migration and diffusion of the condensed sputtered atoms are reasonably small.
4) The atoms arrive at the substrate in random directions due to the discharge gas atoms to cause shadowing effect.
A unit of thin film columnar crystal structure formed due to the above factors is considered to contribute to cause the photomagnetic effect and the magnetic anisotropy in which magnetization of the thin film is aligned perpendicularly to its surface.
In the case where the photomagnetic memory layer is formed by an amorphous alloy thin film having such columnar crystal structure, the photomagnetic property is still not enough for photomagnetic memory medium. Thus, the photomagnetic memory property is improved by adding other elements to the alloy for a multi-element film, or providing a protective film having an enhancing effect over the alloy film for multilayer construction. Although amorphous alloy thin films of such rare-metal transition metals have been studied, their fine structure of the thin film, and chemical, magnetic, and magnetooptics property, the photomagnetic memory medium having adequate CN ratio in practice has not been developed yet. Additionally, since a rare-metal transition metal is easily oxidized, the photomagnetic property thereof has not been sufficiently stable.